CHARACTERISTICS OF THE CONEJOS AQUIFER, THE MAJOR BEDROCK AQUIFER IN THE VOLCANIC ROCKS ABOVE THE LARAMIDE SAN JUAN SAG, EASTERN SAN JUAN MOUNTAINS, RIO GRANDE COUNTY, COLORADO—PART 1: GEOLOGY, WATER QUALITY, AND HETEROGENEITY

The Conejos aquifer supplies ground water to an estimated 500 domestic and public water wells in the mountainous part of Rio Grande County in south-central Colorado. Yet until recently very little was known about the hydrologic character of this regional aquifer. In 2012 a reconnaissance-level hydrogeologic study of this aquifer was conducted by the authors for Rio Grande County in anticipation of renewed interest in oil and gas exploration and production in this area. Funding for the study was provided by the Colorado Water Conservation Board’s State Water Supply Reserve Account through the Rio Grande Basin Roundtable.

Rocks that comprise the Conejos aquifer are included in the early Oligocene Conejos Formation. They range in age from about 33 to 30 Ma and are part of the pre-ash-flow-tuff sequence in the San Juan Volcanic Field, which was active prior to development of the Rio Grande Rift. The aquifer consists of two facies: a vent facies composed chiefly of intermediate-composition lava flows that were erupted from several mid-Tertiary stratovolcanoes, and a volcaniclastic facies that was deposited in more distal locations from the volcanoes. Both facies were east-tilted and locally broken by faulting during rifting. The 2 facies have strikingly different hydrogeologic characteristics. Published geologic mapping and subsurface data from existing oil test wells were used to construct various structure contour and isopach maps.

A baseline water-quality database was developed by sampling 36 domestic water wells and springs within about 1 mile of two proposed oil wells. Samples were analyzed for various constituents, including BTEX and hydrocarbon gasses. None of the sampled wells and springs contained detectable BTEX, and only one spring had detectable hydrocarbon gas (0.012 mg/L of methane; slightly above the Practical Quantitative Limit; probably biogenic gas, not thermogenic). Four of the deepest wells in the area were sampled to assess water quality in the deeper part of the aquifer, and analyses for 2 public water supply wells aided this effort.

Several examples of the heterogeneous character of the aquifer will be described during the talk. Other aspects of the hydrogeologic study, as well as the recommendations made to Rio Grande County, will be covered in the following talk by Harmon and others.